REGULATION OF NEIGHBORING GENE-EXPRESSION BY THE HERPES-SIMPLEX VIRUSTYPE-1 THYMIDINE KINASE GENE

The herpes simplex virus type 1 thymidine kinase (tk) gene (UL23) lies upstream of the gH (UL22) gene with its 3' end overlapping the gH pro moter, and it overlaps the UL24 gene's regulatory and coding sequences at its 5' end in a head-to-head orientation. Thus, tk expression coul d affect gH expression by promoter occlusion and UL24 expression by tr anscriptional or posttranscriptional mechanisms. To investigate these possibilities, we analyzed the effects of tk promoter mutations that r educe tk expression on gH and UL24 expression. For gH, tk promoter mut ations did not increase the accumulation of gH mRNA or the rate of gH transcription. Thus, tk expression does not appear to down-regulate gH expression. In contrast, we found that decreased tk expression correl ated with increased accumulation of UL24 mRNA, particularly a 1.4-kb t ranscript, at early times postinfection during peak expression of tk, but not at late times when tk mRNA levels have fallen. Results from vi ral co-infection experiments indicated that down-regulation of UL24 mR NA accumulation requires tk expression in cis. Nuclear run-off experim ents did not detect differences in UL24 transcription rates in the mut ant viruses. Although we cannot completely exclude a transcriptional m echanism for this down-regulation, these results can be explained by a n antisense RNA mechanism acting preferentially in cis. (C) 1996 Acade mic Press, Inc.